1. Field of Invention
The present invention relates to a free-electron laser (FEL) oscillator which uses an electron beam as a laser medium, particularly for one having an electron beam source constructed by a photoemitter.
2. Related Background Art
The free-electron laser oscillator usually uses the electron beam as the laser medium to attain a laser oscillation by mutual action of the electron beam and an externally applied electric field, magnetic field or laser beam. High current, high brightness and long life of the free-electron source or the electron beam source is desired. Where a linear accelerator is used to accelerate the electron beam, the use of the photoemitter as the electron beam source has been considered in order to produce a bunched beam which conforms to a frequency of a microwave applied to the accelerator. For example, the use of the photoemitter as the electron beam source is discussed in "High-Brightness Photoemitter Injector for Electron Accelerators" by J. S. Fraser et al, IEEE Transactions on Nuclear Science, Vol. NS-32, No. 5 October 1985, and "Pulsed Photocurrents From Lanthanum Hexaboride Cathodes in the ns Regime" by M. Moussoukay et al, Nuclear Instruments and Methods in Physics Research A264 (1988) 131-134, North-Holland Amsterdam.
A principle of operation of the FEL oscillator is explained in detail in "Wiggler and Free Election Laser" by J-Kondo, Study Report No. 200, Electro-Technical Research Lab. AIST, MITI, Japan May 1979. According to the article, the conventional FEL oscillator is constructed as described below. A laser beam from a mode locked YAG laser is reflected by a mirror and directed to a photoemitter in a cavity through a window. Since a target of the photoemitter is made of a material having a photoelectron emitting property such as cecium-antimony, photoelectrons are emitted by the application of the laser beam and the emitted photoelectrons are accelerated by a voltage applied across the photoemitter and an acceleration electrode. The relative theory electron beam which has been accelerated to a vicinity of light velocity by the microwave accelerator has a track thereof changed by a magnetic field generated by a deflection coil and is directed to a periodical magnetic field generated by a helical wiggler.
The laser beam used in the FEL oscillator to emit the photoelectrons is a laser beam having a pulse train of a given duration. It is a pulsed laser beam having a pulse train of 350 pico-seconds (ps) or 770 ps duration which is generated by mode locking in accordance with the period of the microwave used to accelerate the electron beam. As a result, a pulse train of the electron beam corresponding to the pulse train of the laser beam, that is, a bunched electron beam generated from the photoemitter is accelerated by the microwave to become the relative theory electron beam which is close to the light velocity. A method for generating the bunched electron beam is described in "High Quantum Yield from Photofield Emittors" by M. Boussoukaya et al, Nuclear Instruments and Methods in Physics Research A279 (1989) 405-409 North-Holland Amsterdam. When the relative theory electron beam thus generated is directed to the periodic magnetic field generated by the helical wiggler, the track of the electrons is deflected by the periodical magnetic field so that a synchrotron radiation light is generated. Since the synchrotron radiation light interacts with the electrons to generate an electromagnetic field and the electrons interact with the periodic magnetic field, a laser oscillation which uses the free-electrons as a laser medium is started.
The conventional FEL oscillation is generated by a secondary effect of the mutual action of the relative theory electron beam, the periodic magnetic field and the synchrotron radiation light. Accordingly, a high oscillation gain is not attained. In order to attain a high free-electron laser beam output, it has been proposed to use a high brightness electron beam source which permits a high current output of, for example, 50 to 100 A/cm.sup.2. However, in the electron beam source which uses the photoemitter, the damage of the photoemitter is high because a power of the mode locked laser beam irradiated to emit the photoelectrons is high. Accordingly, the lifetime of the photoemitter is reduced to one to several days.
The conventional free-electron laser oscillator uses a condition that the quantity of emission of the synchrotron radiation light from the electrons which are in a deceleration phase in the periodic magnetic field slightly exceeds the quantity of absorption of the light by the electrons which are in an acceleration phase in the same periodic magnetic field, that is, it utilizes the secondary effect of the laser oscillation. Accordingly, if the wavelength of the oscillation laser beam is reduced to attain a high energy, the oscillation gain will further decrease. As a result, the scale of the device of the free-electron laser oscillator for generating the short wavelength laser beam increase.
It is, therefore, a first object of the present invention to provide a free-electron laser oscillator which allows a long life of an electron emission material which forms a target of an electron beam source and assures a high oscillation gain, and an oscillation method therefore.
It is a second object of the present invention to provide a free-electron laser oscillator which allows reduction of the size of the device.